An objective lens with a field of view of about 180 deg (+/−10 deg) is called a fisheye lens. Fisheye lenses are commonly used for security and surveillance applications. New emerging applications include back-up cameras for cars, and interior monitoring cameras for buses and airplanes. Since low light performance of an objective lens is important, the relative aperture of the lens must also be high. Lens relative aperture (also known as f/# of the lens) less or equal to 3 is generally preferred. There is also a need for small size and low weight for many emerging applications. The image quality of the lens must also be high for compatibility with mega pixel class electronic imagers.
Prior art fisheye lenses such as P/N: DF1.8HB marketed by Fujinon in Japan use a large number of lens elements to meet the performance needs (field of view, aperture and high image quality). However, because of the large number of elements used, the lens assembly is large and heavy. They are also expensive to produce due to the use of a large number of lens elements. Examples of prior art fisheye lens designs are U.S. Pat. Nos. 3,589,798, 3,597,049, 3,734,600, 4,412,726, 3,737,214, 3,741,630, 6,844,991, and JP patent 63-017421, and 60-153018. The number of lens elements ranges from 8 to 12 in those designs.